Solving Quadratic Equations

A farmer has 70-acres on which he wants to plant a pear orchard. Three neighboring farms with similar soil conditions already have established orchards. Based on the yield from the neighboring farms, the yield per tree appears to be a linear function of the tree density (trees per acre). Letting t be the number of trees planted per acre, the funtion P(t)= -1.5t + 900 represents the yield in pears per tree.

What function of t would then give the yield per acre for the orchard? Identify the vertex. What is the optimal number of trees to plant per acre?